Metamaterial Optical Filter and Methods for Producing the Same

ABSTRACT

A metamaterial optical filter including: a transparent substrate; and a photosensitive polymer layer provided to the transparent substrate, wherein the photosensitive polymer layer is treated using a laser to form a non-conformal holographically patterned subwavelength grating, the holographic grating configured to block a predetermined wavelength of electromagnetic radiation. A system and method for manufacturing holographically patterned subwavelength grating onto the photosensitive polymer layer including: applying a photosensitive polymer layer to a transparent substrate; placing the photosensitive polymer layer between a laser and a mirror; scanning the laser over the photosensitive polymer layer such that a holographic grating is created within the photosensitive polymer layer by interaction between the laser light and light reflected from the mirror; and stacking two or more holographically patterned subwavelength grating layers to form complex metamaterial optical filter stacks.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.15/653,374, filed Jul. 18, 2017, which claims the priority benefit ofUnited Kingdom Patent Application Number 1613182.3, filed Jul. 29, 2017,the entire contents of both of which are incorporated herein byreference.

FIELD

This document relates to an optical filter and method for producing thesame, and in particular a nano-patterned optically transparent thin filmthat can filter specific narrow optical frequencies.

BACKGROUND

Optical filters are used for many purposes, including for protection ofeyes in various situations. One example of using filters to protect theeyes is in laser applications.

Laser Protection Systems (LPSs) are routinely used in laboratoriesaround the world. LPSs typically come in the form of goggles oreyeshields which are worn by those present during the use of laserradiation. LPSs also come in the form of flat windows which are placedaround the laser location to protect the surroundings. These filters areusually built using polymers and dyes (for low intensity lasers) orglass (for high heat densities).

There are several technical issues regarding currently available LPSsthat affect their feasibility in other applications, such as, forexample, aviation. First, LPSs usually operate to provide protection fora single bandwidth of light such as green or blue, providing protectionfrom a single laser waveband only. Second, LPSs are not generallynarrowband, i.e. limited to just the bandwidth of the laser in use. Thismeans that the LPS will generally block more light than necessary andthus distort the overall vision while reducing the overall lighttransmission. Third, LPSs are usually color tinted, which once againdistorts visual perception. Fourth, glass-based filters are heavy andcan add weight or cannot be comfortably worn by individuals over anextended period of time.

While Bragg-based narrow interference filters made with deposition (e.g.sputtering) have become available in laboratory environments, theseBragg-based narrow interference filters tend to have poor angle opticalperformance, can flake and delaminate, tend to be color tinted and maydistort the visual spectrum.

As such, there is a need for improved optical filters and methods forproducing same.

SUMMARY

According to a first aspect herein, there is provided a metamaterialoptical filter including: a transparent substrate; and a metamaterialfilm provided to the transparent substrate. The metamaterial film isconfigured to block a predetermined bandwidth of electromagneticradiation at a predetermined angle range. The metamaterial film canconsist of one or more photosensitive polymer nano-patterned layersprovided to the transparent substrate, wherein each photosensitivepolymer layer is treated, for example, using a laser, to include aholographically patterned subwavelength grating, the grating includingnon-conformal fringes. The optical filter may be used in variousapplications including aircraft windows (cockpit windshields), trainwindows/windshields, boat windows, car windows, building windows, testequipment windows, eyewear and visors. It will be understood that thepredetermined bandwidth of electromagnetic radiation may be a singlewavelength of electromagnetic radiation. Holographic gratings aresometimes referred to as notch filters.

According to another aspect herein, there is provided a metamaterialoptical filter including: a transparent substrate; and a photosensitivepolymer layer provided to the transparent substrate, wherein thephotosensitive polymer layer includes a holographically patternedsubwavelength grating, the grating including non-conformal fringesconfigured to block a predetermined bandwidth of electromagneticradiation at a predetermined angle.

In a particular case, the holographically patterned subwavelengthgrating may be curved in order to maximize an effective angle ofprotection.

In another case, the holographically patterned subwavelength grating mayinclude a plurality of gratings, wherein each of the gratings may beconfigured to block a different predetermined bandwidth ofelectromagnetic radiation. In this case, at least one of the pluralityof holographically patterned subwavelength gratings may be provided tocolor balance the filter. Further, the plurality of holographicallypatterned subwavelength gratings may be configured to selectively blockat least one of approximately 405 nm, 445 nm, 520 nm, 532 nm, 635 nm,650 nm wavelengths.

In yet another case, the photosensitive polymer layer may be shaped forthe transparent substrate using thermoforming and the photosensitivepolymer layer may be pre-configured to allow for changes to thephotosensitive polymer layer during thermoforming.

In still another case, the photosensitive polymer layer may be infusedwith a dye selected to color balance the filter.

In still another case, the metamaterial optical filter may furtherinclude a supplemental substrate that contains a dye selected to colorbalance the filter.

In yet still another case, the transparent substrate may include two ormore transparent substrates and the photosensitive polymer film may bepositioned between at least two of the two or more transparentsubstrates.

In yet another case, the predetermined angle of the non-conformalfringes may be up to 75 degrees below a normal axis of the filter.

In still another case, the electromagnetic radiation is opticalradiation from a laser.

In yet still another case, metamaterial optical filter may furtherinclude an adhesive to bond the transparent substrate and photosensitivepolymer layer or photosensitive polymer layers, wherein the adhesive mayinclude graphene.

In still another case, the transparent substrate may selected from oneof the following a window, eyewear, and a visor.

According to another aspect herein, there is provided a system formanufacturing a metamaterial optical filter, the system including: aclamp for holding a photosensitive polymer layer applied to a mirror; alaser; and a laser transport system for moving the laser relative to thephotosensitive polymer film such that, as the laser moves over a surfaceof the photosensitive polymer film, laser light is reflected off of themirror to create a holographically patterned subwavelength gratingswithin the photosensitive polymer layer.

In a particular case, the laser transport system may include: a carriagefor carrying the laser; and rails provided adjacent the surface of thephotosensitive polymer layer such that the carriage movably engages withthe rails and is configured to move across the surface of thephotosensitive polymer layer. In this case, the carriage and rails maybe configured to move the laser in the longitudinal direction and theclamp may be configured to move the photosensitive polymer layer in thelatitudinal direction.

According to another aspect herein, there is provided a method ofmanufacturing a metamaterial optical filter, the method including:applying a photosensitive polymer layer to a substrate; placing thephotosensitive polymer layer between a laser and a mirror; and scanningthe laser over the photosensitive polymer film such that aholographically patterned subwavelength grating is created within thephotosensitive polymer layer by interaction between the laser light andlight reflected from the mirror.

In a particular case, the scanning may include moving one or more of thelaser, the photosensitive polymer layer and the mirror.

In another case, the method for manufacturing may further includethermoforming the photosensitive polymer layer to have a repeatablefilter wavelength shift and a filter wavelength pre-compensation of thephotosensitive polymer layer such that the thermoformed photosensitivepolymer layer meets predetermined requirements. In this case, theoriginal photosensitive polymer layer's bandgap may be pre-shifted tolonger wavelengths in order to counter-balance the shift caused by thethermoforming process. Further, the bandgap pre-shift may be radiallydependent with gradually smaller shifting away from the center of thephotosensitive polymer layer.

In yet another case, the laser may be split into separate beams and thebeams may be directed onto the photosensitive polymer layer at differentangles of incidence.

In still another case, the laser may include a plurality of lasers ofdifferent wavelengths that are combined into a single combined beam andthe combined beam may be directed onto the photosensitive polymer layerat a predetermined angle of incidence allowing notch filters ofdifferent wavelengths to be recorded simultaneously.

In yet still another case, each holographically patterned subwavelengthgrating formed on a single photosensitive polymer layer may bere-combined into a multi-layered metamaterial optical filter stack,including at least two or more holographically patterned gratings toallow control of angle, bandgap, optical density and color balance ofthe multilayered metamaterial optical filter stack. In this case, themethod for manufacturing may further include an adhesive to bond themulti-layered metamaterial optical filter stack wherein the adhesive mayinclude graphene.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures illustrate various aspects and embodiments of themetamaterial filter and system and method of manufacture herein.

FIG. 1 illustrates an embodiment of a metamaterial film applied to anaircraft windshield;

FIG. 2 illustrates a principle of a reflective filter made using agrating;

FIGS. 3A, 3B and 3C illustrate different types of holographicallypatterned subwavelength gratings;

FIG. 4 illustrates angle coverage achieved with an example ofholographically patterned subwavelength curved grating on aphotosensitive polymer layer;

FIG. 5 illustrates horizontal angle coverage for a first aircraftwindow;

FIG. 6 illustrates horizontal angle coverage for a second aircraftwindow;

FIG. 7 illustrates horizontal angle coverage for a third aircraftwindow;

FIG. 8 illustrates vertical angle coverage for a first aircraft window;

FIG. 9 illustrates vertical angle coverage for a second and thirdaircraft window;

FIG. 10 illustrates a cross-section of a thermoformable metamaterialmultilayer optical filter;

FIG. 11 is a schematic of an embodiment of a system for recording acurved grating;

FIG. 12 is a schematic drawing of a system for manufacturing ametamaterial film according to an embodiment herein;

FIG. 13 is a schematic drawing of a system for manufacturing ametamaterial film according to another embodiment herein;

FIG. 14 is a schematic drawing of a system for manufacturing ametamaterial film according to still another embodiment herein;

FIG. 15 is a schematic drawing of a system for manufacturing ametamaterial film according to still another embodiment herein;

FIG. 16 is an illustration of a multiple stacked photosensitive polymerlayers with multiple holographically patterned subwavelength-gratingsforming the metamaterial film;

FIG. 17 is a transmittance spectrum for a metamaterial film having onegrating; and

FIG. 18 is a transmittance spectrum for a metamaterial film havingmultiple gratings;

FIG. 19 illustrates various components of an aircraft windshieldaccording to an embodiment herein; and

FIG. 20 illustrates various components of an aircraft windshieldaccording to another embodiment herein.

DETAILED DESCRIPTION

Metamaterials have been proposed as a new type of optical filter forvarious applications (see, for example PCT Publication No.WO/2013/054115, application filed Oct. 10, 2012). Nano-patternedmulti-layered gratings can be designed for the development oftransparent, ultra-thin optical filters. One additional example of ametamaterial film, in addition to those disclosed in the noted PCTapplication is a photosensitive polymer layer based on the Covestro™Bayfol™ material, or one of its derivatives.

These metamaterial filters can be applied to aircraft cockpitwindscreens in order to selectively block light from, for example, laserattacks, which are a growing concern in the aviation industry. A brightvisible laser light causes distraction or temporary flash blindness to apilot, during a critical phase of flight such as landing or takeoff. Itis far less likely, though still possible, that a visible or invisiblebeam could cause permanent harm to a pilot's eyes. These metamaterialfilters have the advantage that only a narrow band of light is blockedso that the aircraft crew retain good visibility through the windscreenwhile being protected from a possible attack especially during nighttime operation.

FIG. 1 shows an example of a metamaterial formed as a nano-patterned,optically transparent film 100 that is applied to a windshield in anaircraft application. The metamaterial film 100 is typically applied ona surface of the glass cockpit window 105 of an airplane 110. It will beunderstood that the embodiments of the metamaterial filter and systemand method herein may also be applied to or used in various applicationsincluding: eyewear, visors for helmets, heads up displays, or the like;windows of houses, buildings or the like; as well as with windscreens orwindshields of other vehicles including cars, trucks, trains, boats,other aircraft such as helicopters, unmanned air vehicles, and the like.Further, in some applications the metamaterial film may be appliedbetween glass panes/layers of a multi-layer windscreen, window, eyewear,visor or the like.

The metamaterial optical filter generally includes an optically clearadhesive film layer that can be applied on to the surfaces ofeyeglasses, cockpit windscreens and other transparent surfaces. Themetamaterial filter is generally transparent to all incoming light inthe visible spectrum with the exception of certain wavelengths that aresimultaneously and largely reflected by, and attenuated or partiallyabsorbed inside by, the filter layers and its subwavelength gratings,protecting the persons behind the filter. The filters provide protectionfrom electromagnetic radiation, such as optical spectrum wavelengths,including laser wavelengths, in a passive manner (i.e. without requiringa power source).

One of the difficulties with optical filters of the type described aboverelates to the complexity of making the thin film filters large enoughand easily integrated for various applications such as windscreens andthe like. One particular method for creating the nano-patternedsub-wavelength gratings inside the metamaterial film is usingholography. Holography or photorefraction offers the possibility of afabrication method where even complex large area films can bemanufactured quickly, for example, in minutes.

A photorefractive material has the property that its refractive indexcan be modified based on incident laser radiation. In the simplestscenario, the sample is placed against a mirror and illuminated with alaser. The interference intensity pattern created inside thephotorefractive material causes changes in its refractive index, whichcan be tuned to match a desired refractive index pattern, creating aBragg type mirror or Bragg grating. FIG. 2 illustrates the principleinvolved in creating the interaction of a Bragg type mirror with alaser. The Bragg type mirror/grating 115 is depicted as alternatinglayers of high 120 and low 125 refractive index gratings ofpredetermined thicknesses (based on the wavelength of radiation to beblocked). Typically, dozens or hundreds of layers are required, but onlya few are shown in this illustration for clarity. Once the Bragg gratinghas been created, when a laser beam 130 is incident at an angle that isclose to perpendicular to the Bragg grating direction, constructiveinterference effects cause the laser beam to be reflected back 135 (i.e.filtered).

Further, metamaterial filters made using holographically patternedsubwavelength gratings can offer the wavelength selectivity of adielectric filter, with the added advantage of a continuously-variablerefractive index in the photosensitive polymer layers. This property isexpected to increase the efficiency and wavelength selectivity.Metamaterial filters made using holographically patterned subwavelengthgratings also offer dielectric layers that can tilt or curve withoptical power normally only seen in refractive optics. Metamaterialnotch filters can be optical elements that also bend or focus light.

Photosensitive polymer layers in particular can be made in thick layers(greater than 6 microns) because they require no access for wetdeveloping chemicals; thicker layers can mean greater efficiency andeven narrower wavelength selectivity (narrower bandwidth). Metamaterialfilters made using multi-layered holographically patterned subwavelengthgratings appear to offer the ability to filter out the near-theoreticalminimum bandwidth to block a laser line and therefore exhibit lowestimpact on the overall transmitted spectrum and color perception.

Metamaterial filters can be formed as parallel-fringe holographicallypatterned subwavelength gratings or as slanted-fringe holographicallypatterned subwavelength gratings. Both may be effective as tunablefilters. However, from the point of view of mass production, there aremajor practical differences between making a slanted-fringe grating anda non-slanted one (where the fringe planes lie parallel to the surface).

Slanted-fringe gratings require either the presence of the fullarrangement of optical components needed to generate the specificwavefronts required, or the ability to ‘fix’ this arrangement once ithas been assembled in, for example, the laboratory. This requirement tohave the full arrangement available is typically achieved by recordingit in a ‘master’ grating layer that is then used to make further contactcopies.

Slanted-fringe grating manufacturing could be done for a part such as a‘goggle lens’ with the lens itself manufactured complete with anattached layer of unexposed photosensitive polymer in the productionmachine. Optical components could generate the necessary and possiblycomplex reference and subject beams and a robotic mechanism wouldpresent the part to the exposure station and remove it for furthersteps. Another possibility is to bring the blank lens together with itsphotosensitive polymer laminate—before or after exposure—within themanufacturing machine. Still further integration may be possible if theproduction machine molds the lens in plastic while the photosensitivepolymer layer is inserted within it or applied after cooling. Anintegrated approach may be necessary if the photosensitive polymer layerneeds to be curved in one or two dimensions either during or afterrecording (or both).

If the slanted-fringe grating can be made as an original ‘master’grating layer, then it can be copied optically by scanning it with asingle line of laser light running across the direction of the ‘web’.This would be used if the master grating layer is either wrapped aroundthe outside of a solid drum (if a reflection grating) or on the outsideof a transparent drum illuminated by a line of laser light from theinside (if a transmission grating).

Scanning by a line of laser light is also sometimes employed when theslanted-fringe ‘master’ grating layer is copied lying flat, which by itsnature requires that a step and repeat procedure is used. But if stepand repeat is used—that is, the master and the copy are fixed in spaceover their entire surface area for a substantial time—then the wholesurface of the master grating layer can be illuminated at the same time,limited only by the power of the laser.

In either case, a flat master grating layer copied by step and repeatmay be needed for very large-area metamaterial filters, such as for anaircraft windshield.

Non-slanted fringe gratings for notch filters, also called ‘holographicmirrors’ because of both their appearance and the way they are made, aresimpler to manufacture because they can tolerate motion in the plane ofthe recording medium during the exposure: specifically, if the recordingmedium moves parallel to the fringe planes during exposure, then thefringes are not blurred and can still be recorded with good contrast andgrating efficiency. In practice, a machine to make such a nano-patternedsubwavelength grating can tolerate ‘slippage’ of the photosensitivepolymer layer in its own plane and this property has been usedindustrially to manufacture large-area seamless notch filters where thelaser and a flat mirror behind the photosensitive polymer layer create a‘stack’ of local interference fringes through which the moving webintentionally passes through.

In manufacturing volume gratings, the complex arrangement of manyoptical components on a vibration-isolated table can sometimes bereplaced by a ‘master’ grating layer that is copied by a single expandedlaser beam or a scanned line of laser light. Typically, an efficienthologram is needed for a ‘reflection’ grating, or a weak one (50%efficiency) for a transmission grating, but in either case it will benecessary that the grating can be copied by a collimated beam of laserlight, which may be a limitation for complex metamaterial notch filterdesigns.

To manufacture relatively thick, so-called ‘volume grating, it isnecessary to establish an optical interference pattern stably fixedwithin the photosensitive polymer layer for the duration of the exposuretime. A volume grating gets its name because it is thick enough torecord a three-dimensional volume of interference fringes: for example,a 14-micrometer thick photosensitive polymer layer can record more thantwo dozen half-wave fringe layers. Unlike the mechanical embossingprocess used to reproduce the silver security holograms on credit cards,a coherent illumination source, such as a laser, is a necessary part ofthe production line to generate the ‘live’ interference pattern.Laboratory lasers are readily available with the necessary opticalproperties to make metamaterial filters using holographically patternedsubwavelength gratings, but the finite limit on the power of actuallasers may set the upper limit on the speed of any manufacturingprocess.

The recording medium should preferably have high spatial resolution, forexample, from approximately 1000 lines per mm and higher, and low noiseor scattering so the resulting metamaterial filter has a glassytransparency. On the production line, a degree of mechanical stabilityis required during the moments of exposure. The material should besufficiently sensitive that the effective exposure time can be shortenough to ensure no movement on scales less than the wavelength oflight.

Most nano-patterned and/or Bragg-style laser filters have inherentlynarrow operation in terms of the angle of the incident laser light. Thisis because the angle of operation is linked to the bandgap of the filterfor a traditional periodic (high and low index) design: as the bandgapis reduced, the angle of incidence (AOI) for which the bandgap appearsis reduced as well. Interestingly, this issue can be solved by curvingthe gratings of the metamaterial film such that their radius ofcurvature is centered at the location where the eye receiving radiationis expected to be found. This can be achieved by placing thephotosensitive polymer layer on a curved substrate mold (with, forexample, radius of curvature Rm) that may sit on top of a flat mirror.The substrate mold should be index-matched to the photosensitive polymerto provide maximum grating formation efficiency and suppress anyunwanted effects. If the polymer then is bent by a radius of curvatureRp, the effective radius of curvature will be 1/(1/Rp−n/Rm), n being theaverage refractive index of the photosensitive polymer layer.

FIG. 3 shows various types of grating layers: a parallel (non-slanted orconformal) grating (FIG. 3A); a slanted (fixed-slant) grating (FIG. 3B)and a curved grating (FIG. 3C). For a curved grating layer, theholographically patterned subwavelength curved grating may be formedsuch that, on a flat surface, the grating has a main axis (k-vector)that varies in angle as a function of the position along themetamaterial film. For example, for each horizontal location in themetamaterial film (x-axis) a 40° AOI may be achieved (where the opticaldensity is greater than 2.0) but this AOI will be centered around adifferent angle along the length of the metamaterial film. The curvedgrating allows for a larger protection angle, as illustrated in FIG. 4,where more than 60° angle coverage is achieved with an example curvedgrating layer.

For the case of a vehicle windscreen such as an aircraft windscreen, thecentral point between the two eyes can be used to align the radius ofcurvature of the films. Further, in a vehicle application, the gratingsof the metamaterial film can be layered and slanted at various angles,so that they can be optimized to filter laser radiation from aparticular direction, typically originating from the ground. FIGS. 5 to9 illustrate example coverage angles (both horizontal and vertical) forairplane cockpit windows. FIG. 5 shows horizontal angle coverage A for afirst window (window 1) of an aircraft. For a particular example of anAirbus™ A330™ aircraft, the angle A may be 85° with the angles relativeto an axis X of −15° to +70°. FIG. 6 shows horizontal angle coverage Bfor a second window (window 2) of an aircraft. For the particularexample, the angle B is 90° with the angles relative to the axis X of−50° to +40°. FIG. 7 shows horizontal angle coverage C for a thirdwindow (window 3) of an aircraft. For the particular example, the angleC is 28° with the angles relative to the axis X of −72° to +45°. FIG. 8shows vertical angle coverage D and E for the first window (window 1).In an aircraft situation, the coverage is generally only needed belowthe horizon, and particularly with angle D between −7° to −63° (D=E inthis case). FIG. 9 shows vertical angle coverage F and G for the secondand third windows (windows 2 and 3). For the particular example of anaircraft, the required filtering coverage (elevation angle F) in thesecases is +12° to −55° for window 2 and +9° to −40° for window 3. Theangles mentioned here are intended to take into consideration also theaeronautic behavior of the aircraft during the different phases offlight, as well as the position of potential laser sources originatingfrom the ground.

While metamaterial films can generally be easily laminated to flat andcylindrically curved surfaces, metamaterial films are generally moredifficult to laminate to objects having compound (spherical) curvaturesuch as a lens, helmet visor or windscreen. This problem is akin towrapping a ball with a flat piece of paper. Thermoforming provides apotential solution to the technical challenge.

A method according to an embodiment herein is to first laminate the oneor more metamaterial filters made using holographically patternedsubwavelength gratings to a flat substrate e.g. polycarbonate or glass,using an optical adhesive. Both the metamaterial filter and adhesivefilm can be composed of thermoformable layers e.g. silicone, polymer,etc. This adhesive-metamaterial stack is then cut via a machine to thedesired pre-formed shape e.g. a 75 mm diameter circle to form a lens.The stack is then thermoformed via controlled pressure and temperatureinto the desired geometry e.g. spherical curvature. Theadhesive-metamaterial stack and selected substrate are then of sphericalcurvature and are able to use standard lens finishing processes tointegrate into non-flat laser protective eyewear lenses, canopies,windshields or the like. In manufacturing, the metamaterial filterwavelengths can be pre-compensated for the physical stretching duringfilter formation. FIG. 10 shows a cross-section of a photosensitivepolymer layer before thermoforming (with vertical axis enlarged forclarity) that illustrates an example of this type of pre-compensation.In particular, the gratings at the center of the photosensitive polymerlayer have a smaller spacing (period) than the gratings at the edge ofthe photosensitive polymer layer, which have a larger spacing (period).When the metamaterial film is thermoformed, the edges will be stretchedand the grating spacing will become uniform on the lens, leading tofiltering (laser blocking) of the same wavelength and bandgap across thecurved surface. In this way, during the thermoforming process, the filmcan have a repeatable filter wavelength shift with wavelengthpre-compensation of the film such that thermoformed film meetspredetermined blocking/filtering requirements. For example, beforethermoforming is to be applied, the original metamaterial film's bandgapcan be pre-shifted to shorter or longer wavelengths in order tocounter-balance the shift caused by the thermoforming process. Further,the bandgap pre-shift of the to-be-thermoformed photosensitive polymerlayer does not need to be uniform; but can be radially dependent withgradually smaller shifting away from the center of the film. As such, inthe case of a simple lens, a thermoforming process can create a radialmetamaterial filter with wavelength dependence to enhance the overallblocking angle. In this case, the shortest wavelengths are in the centerof the photosensitive polymer layer and the longer wavelengths are atthe periphery.

In a similar way to the pre-compensation for thermoforming, themetamaterial filter may also be pre-compensated based on the intendedoperating conditions of the environment where the filter will beinstalled. For example, some aircraft windshields have heating systemsand temperature may impact the configuration of the gratings such thatthe filter can be configured to provide the desired characteristicsbased on the intended operating environment.

FIG. 11 illustrates an embodiment of a system 1000 and method forrecording a curved grating on a photosensitive polymer layer. Inparticular, a photosensitive polymer layer 1005 is held in a curved mold1010 including a first part 1015 and a second mold part 1020. The curvedmold could be a cylindrical mold or a spherical mold depending on thetype of curvature desired in the grating, i.e. depending on whether thecurvature is intended to be two-dimensional or three-dimensional. Thecurved mold 1010 is placed against a flat mirror 1025. An incident laserbeam 1030 is then scanned over the photosensitive polymer layer 1005 andthe reflected beam 1035 returns from the mirror to create a reflectiongrating within the photosensitive polymer layer 1005 from theinterference between the incident and reflected beams. In this way,planar interference fringes and a grating are formed within thephotosensitive polymer layer region, but since the photosensitivepolymer layer is bent, when the photosensitive polymer layer is removedfrom the mold and flattened these fringes will become curved, as shownin FIG. 3C referred to above.

In order to manufacture the above-described metamaterial filters usingholographically patterned subwavelength gratings, different principlescan be used. For example, in one embodiment, the exposure to the lasermay be made by a moving head carrying the laser or its output through aflexible fiber optic cable, moving over the width or length of thephotosensitive polymer layer on a rail, and being scanned by successiveoffset lines. The exposure may be made directly, in a narrow line acrossthe width or length of the photosensitive polymer layer web, by thepassage of the laser head on a carriage powered by, for example, aflexible drive belt on a steel guide rail. The width of the beam, thepower of the laser and the distance the photosensitive polymer layer webis advanced between exposures can be determined empirically to achieve aproduct of high optical density with no visible lines. The laser outputhead may also be mounted on a motorized rotation stage, so the angle ofincidence can be set to compensate the shrinkage of a particular batchof photosensitive polymer layer or expansion created byadhesives/processing to be used in the windshield.

FIG. 12 is a schematic diagram of a system 1100 for manufacturing ametamaterial film filter including one or multiple-layers of complex andconformal gratings. In FIG. 12, a photosensitive polymer layer on atransparent substrate 1105 is adjacent to or applied to a mirror orreflective film 1110. A laser 1115 is attached to a carriage 1120provided to one or more rails 1125 such that the laser 1115 isconfigured to move along a longitudinal direction of the photosensitivepolymer layer 1105 to process one “line” of the photosensitive polymerlayer 1105. In this particular embodiment, a plurality of (three) lasers1115 and carriages 1120 are provided and in other embodiments a greateror lesser number of lasers and associated equipment may be provided.After completing one pass or “line”, the laser 1115 (on carriage 1120)then returns to the beginning of the rails 1125 and the photosensitivepolymer layer 1105 is moved perpendicular to the direction of motion ofthe carriage 1120 so that the laser 1115 can process an additional“line” of the photosensitive polymer layer 1105. In this way, thephotosensitive polymer layer 1105 is treated by the laser 1115 at anappropriate level of resolution. In this embodiment, the carriage 1120also includes a rotation head 1130 configured to allow the laser 1115 torotate so that the angle of incidence on the photosensitive polymerlayer can be adjusted depending on the desired fringe angle. Therotation of the laser may be performed in advance of laser scanning ormay be performed during laser scanning in order to adjust the fringeangle for different areas of the photosensitive polymer layer.

FIG. 13 illustrates another embodiment of the system for manufacturing1200. In this embodiment, the principle is similar to the embodimentshown in FIG. 12 but with the difference that the laser 1115 includesoptional lens 1135 that diverge the laser beam into a ‘fan’ of lightaligned with the direction of travel of the carriage 1120. This “fan” oflight creates exposures over a range of angles, inducing a broaderangular, and therefore spectral, bandwidth in the resulting filter. Thelens 1135 thus provides a control mechanism to vary filter bandwidth.

FIG. 14 is a schematic diagram showing a further embodiment of a systemfor manufacturing 1300 similar to that in FIG. 13. In this particularembodiment, a plurality of (three) lasers 1115 with lenses 1135 areprovided. The use of multiple lasers allows for the recording ofmultiple gratings on a single photosensitive polymer layer (notchfilters) that may, for example, be configured to block differentwavelengths. As an example, red, green and blue lasers may be used. Itwill be understood that, in other embodiments, a greater or lessernumber of lasers and associated equipment may be provided.

When using a plurality of lasers, a plurality of notch filters eachhaving different effective wavelengths can be provided to themetamaterial film stack through the process of recording the reflectiongratings consecutively or simultaneously using the plurality of laserbeams. Although not shown in FIG. 14, the notch filters may be recordedsimultaneously by splitting a single recording laser into separate beamsand directing said beams onto the photosensitive polymer layer atdifferent angles of incidence. Alternatively, a plurality of lasers ofdifferent wavelengths may be combined into a single beam and thecombined beam is then directed onto the photosensitive polymer layer atthe desired angle of incidence allowing notches of different wavelengthsto be recorded simultaneously. Each recorded grating layers with one ormultiple notches may be combined by stacking one or more of thesegrating layers on top of each other to create a complex metamaterialfilter stack with multiple notches of different wavelengths, angle,color and optical density control.

In these embodiments, the ‘subject beam’ for the grating may be suppliedby i) internal surface reflection from the final surface of thephotosensitive polymer layer or ii) a reflective film or mirror (such asmirror 1110 shown in FIG. 12) temporarily provided to the photosensitivepolymer layer, for example by lamination or the like, or iii) a solidmirrored surface such as a metal mirror or coated glass mirror, with thephotosensitive polymer layer in optical contact by means of an indexmatching liquid or the like.

FIG. 15 illustrates an embodiment of a system for manufacturing 1400that is configured to provide a slanted-fringe grating. In FIG. 14, thephotosensitive polymer layer 1105 is provided on a transparent substrate1136. The transparent substrate 1136 could be, for example, Polymethylmethacrylate (PMMA), polycarbonate, triacetate (TAC), or the like. Laser1115 is again attached to carriage 1120 for movement in relation to thephotosensitive polymer layer 1105, however, in this embodiment,differing from the embodiments in FIGS. 12 to 14, rails 1140 run in alateral direction to the photosensitive polymer layer 1105 and thephotosensitive polymer layer 1105 is movable in the horizontal directionshown by the arrow 1145. Further, in this embodiment, a mirror 1150 isprovided to a second carriage 1155 provided on an opposite side of thephotosensitive polymer layer 1105 from the laser 1115. The secondcarriage 1155 includes a second rotation head 1160 allowing the mirror1150 to rotate in coordination with the laser 1115 so that the mirror1150 is angled towards the laser 1115. In this embodiment, liveinterference fringes are generated by having the mirror 1150 configuredto ‘retroreflect’ the laser beam incident from the laser 1115 onto thetop side of the photosensitive polymer layer 1105. Slanted-fringes willbe generated where the incident and retroreflection beams cross 1165.

FIG. 16 illustrates the principle of a multi-wavelength metamaterialfilter stack 1500. In this stack, there are three recorded gratings ofthree photosensitive polymer layers, each having a different spacingthat is configured to block a different wavelength or wavelength range.In particular, a first layer 1505 is configured to block a firstwavelength 1510, a second layer 1515 is configured to block a secondwavelength 1520, and a third layer 1525 is configured to block a thirdwavelength 1530. It will also be understood that the gratings/layerscould also overlap in space by using appropriate materials. Thus use ofstacked recorded photosensitive polymer layers can increase theeffective protection angle and/or negate dispersion effects caused byindividual layers and/or improve optical density of the filter. When apair of slanted grating films are stacked together, these films willgenerally have the opposite slant.

In addition to the photosensitive polymer grating layers, themetamaterial filter stack may also include other substrate or supportinglayers. As noted herein, the substrate or supporting layers may includePMMA, polycarbonate, triacetate (TAC) or the like. Various combinationsof photosensitive polymer (PP) grating layers and substrate may beformed, in this case using TAC for example: TAC-PP-PP-TAC;TAC-adhesive-PP-PP-adhesive-TAC;TAC-adhesive-PP-adhesive-PP-adhesive-TAC and the like. The metamaterialfilter stack may also include films or coatings for anti-glare,anti-scratch, glazing or the like. The adhesive used in the stack mayany appropriate adhesive and may also include graphene.

In some embodiments, a metamaterial filter may be improved by alsoproviding saturation reduction/minimization and/or color balancing forthe filter. In particular, a Bragg-type subwavelength holographicallypatterned grating creates a bandgap and reflects a section of thespectrum in order to block predetermined wavelengths, such as those froma laser. However, this process generally distorts the neutral color of ascene as viewed through such a grating. For example, a grating thatblocks 532 nm light will generally appear pink since a green portion ofthe spectrum is removed. This effect can be counter balanced (colorbalanced) by also removing some red and blue portion of the spectrum, toproduce a color neutral result. This color balancing result can beachieved by adding absorbing dyes, or by adding more holographicallypatterned gratings that filter out blue and red portions of thespectrum. FIG. 17 shows an example of the transmittance spectrum of atwo-layer metamaterial color balanced filter stack, a first layer beinga holographically patterned grating at 532 nm and the second layer beinga polycarbonate sheet doped with blue and red absorbing dyes withpredetermined concentrations to balance the color of the overallmetamaterial filter stack. FIG. 18 shows an example of the predictedtransmittance spectrum when a metamaterial color balanced filter stackusing three holographically patterned gratings (blue, green and red) areused (as illustrated in FIG. 16). The resulting metamaterial filterstack exhibits excellent color balancing properties and, in addition tothe green laser (similar to the filter of FIG. 17), this type of filtercan also can block red and blue lasers (these wavelengths are shown withvertical lines).

As noted herein, the metamaterial film may be used/applied in variouslocations in relation to other components of an aircraft windshield. Insome cases, the metamaterial may be used/applied on the interior of thewindshield while in others, the metamaterial may be used/applied withina windshield structure. FIG. 19 and FIG. 20 show two alternateconfigurations of windshields.

In the preceding description, for purposes of explanation, numerousdetails are set forth in order to provide a thorough understanding ofthe embodiments. However, it will be apparent to one skilled in the artthat these specific details may not be required. In other instances,well-known structures may be shown in block diagram form in order not toobscure the understanding. For example, specific details are notprovided as to whether elements of the embodiments described herein areimplemented as a software routine, hardware circuit, firmware, or acombination thereof.

Embodiments of the disclosure or components thereof can be provided asor represented as a computer program product stored in amachine-readable medium (also referred to as a computer-readable medium,a processor-readable medium, or a computer usable medium having acomputer-readable program code embodied therein). The machine-readablemedium can be any suitable tangible, non-transitory medium, includingmagnetic, optical, or electrical storage medium including a diskette,compact disk read only memory (CD-ROM), memory device (volatile ornon-volatile), or similar storage mechanism. The machine-readable mediumcan contain various sets of instructions, code sequences, configurationinformation, or other data, which, when executed, cause a processor orcontroller to perform steps in a method according to an embodiment ofthe disclosure. Those of ordinary skill in the art will appreciate thatother instructions and operations necessary to implement the describedimplementations can also be stored on the machine-readable medium. Theinstructions stored on the machine-readable medium can be executed by aprocessor, controller or other suitable processing device, and caninterface with circuitry to perform the described tasks.

The above-described embodiments are intended to be examples only.Alterations, modifications and variations can be effected to theparticular embodiments by those of skill in the art without departingfrom the scope, which is defined solely by the claims appended hereto.

1-25. (canceled)
 26. A method of manufacturing a metamaterial opticalfilter configured to block a predetermined bandwidth of electromagneticradiation at a predetermined angle, the method comprising: providing aphotosensitive polymer layer disposed between a laser and a mirror; andscanning the laser over the photosensitive polymer layer such that aholographically patterned subwavelength grating is created within thephotosensitive polymer layer by interaction between the laser light andlight reflected from the mirror, the holographically patternedsubwavelength grating having a spacing dependent on the predeterminedbandwidth of electromagnetic radiation; wherein the scanning includesscanning over a first portion of the surface of the photosensitivepolymer layer and scanning over a second, different portion of thesurface of the photosensitive polymer layer.
 27. The method according toclaim 26, wherein the scanning includes scanning over successive offsetlines across the surface of the photosensitive polymer layer.
 28. Themethod according to claim 26, wherein the scanning includes moving thelaser in a longitudinal direction and moving the photosensitive polymerlayer in a latitudinal direction.
 29. The method according to claim 26,wherein the scanning comprises moving one or more of the laser, thephotosensitive polymer layer and the mirror.
 30. The method according toclaim 26, further comprising thermoforming the photosensitive polymerlayer to have a repeatable filter wavelength shift and a filterwavelength pre-compensation of the photosensitive polymer layer suchthat the thermoformed photosensitive polymer layer meets predeterminedrequirements.
 31. The method according to claim 30, wherein the bandgapof the photosensitive polymer layer before thermoforming is pre-shiftedto longer wavelengths in order to counter-balance the shift caused bythe thermoforming.
 32. The method according to claim 31, wherein thebandgap pre-shift is radially-dependent, with gradually smaller shiftingaway from a center of the photosensitive polymer layer.
 33. The methodaccording to claim 26, wherein the laser is split into separate beamsand the beams are directed onto the photosensitive polymer layer atdifferent angles of incidence.
 34. The method according to claim 26,wherein the laser comprises a plurality of lasers of differentwavelengths that are combined into a single combined beam and thecombined beam is directed onto the photosensitive polymer layer at apredetermined angle of incidence, thereby recording notch filters ofdifferent wavelengths simultaneously.
 35. The method for manufacturingaccording to claim 26, comprising forming a holographically patternedsubwavelength grating in each of a plurality of photosensitive polymerlayers, and wherein the plurality of photosensitive polymer layers arecombined into a multi-layered metamaterial optical filter stack,comprising at least two or more holographically patterned gratings toallow control of angle, bandgap, optical density and color balance ofthe multi-layered metamaterial optical filter stack.
 36. The method formanufacturing according to claim 35, wherein an adhesive comprisinggraphene is used to bond the plurality of photosensitive polymer layersof the multi-layered metamaterial optical filter stack.
 37. The methodaccording to claim 26, wherein the photosensitive polymer layer isdisposed on a substrate and the photosensitive polymer layer disposed onthe substrate is placed between the laser and the mirror.
 38. The methodaccording to claim 26, wherein the holographically patternedsubwavelength grating comprises non-conformal fringes configured toblock the predetermined bandwidth of electromagnetic radiation at thepredetermined angle.
 39. The method according to claim 38, wherein theholographically patterned subwavelength grating of the optical filter iscurved in order to increase an effective angle of protection.
 40. Themethod for manufacturing according to claim 38, wherein theholographically patterned subwavelength grating comprises a plurality ofgratings, wherein each of the gratings is configured to block adifferent predetermined bandwidth of electromagnetic radiation.
 41. Themethod of manufacturing according to claim 40, wherein at least one ofthe plurality of holographically patterned subwavelength gratings of theoptical filter is provided to color balance the filter.
 42. The methodof manufacturing according to claim 40, wherein the plurality ofholographically patterned subwavelength gratings of the optical filterare configured to selectively block at least one of approximately 405nm, 445 nm, 520 nm, 532 nm, 635 nm, 650 nm wavelengths.
 43. The methodof manufacturing according to claim 38, wherein the photosensitivepolymer layer of the optical filter is shaped for a transparentsubstrate using thermoforming and the photosensitive polymer layer ispre-configured to allow for changes to the photosensitive polymer layerduring thermoforming.
 44. The method of manufacturing according to claim38, wherein the photosensitive polymer layer is infused with a dyeselected to color balance the filter.
 45. The method of manufacturingaccording to claim 38, wherein the optical filter includes a substratethat contains a dye selected to color balance the filter.
 46. The methodof manufacturing according to claim 38, wherein the optical filtercomprises two or more transparent substrates and the photosensitivepolymer layer is positioned between at least two of the two or moretransparent substrates.
 47. The method of manufacturing according toclaim 38, wherein the predetermined angle of the non-conformal fringesis up to 75 degrees below a normal axis of the filter.
 48. The method ofmanufacturing according to claim 38, wherein the optical filter furthercomprises a transparent substrate and an adhesive bonding thetransparent substrate and photosensitive polymer layer wherein theadhesive comprises graphene.
 49. The method of manufacturing accordingto claim 38, wherein the optical filter includes a transparent substrateon which the photosensitive polymer layer is disposed.
 50. The method ofmanufacturing according to claim 49, wherein the transparent substrateis selected from one of the following: a window; eyewear; and a visor.51. A system for manufacturing a metamaterial optical filter, the systemcomprising: a clamp for holding a photosensitive polymer layer appliedto a mirror; a laser; and a laser transport system for moving the laserrelative to the photosensitive polymer layer such that, as the lasermoves over a surface of the photosensitive polymer layer, laser light isreflected off of the mirror to create a holographically patternedsubwavelength gratings within the photosensitive polymer layer.
 52. Thesystem according to claim 51, wherein the laser transport systemcomprises: a carriage for carrying the laser; and rails providedadjacent the surface of the photosensitive polymer layer such that thecarriage movably engages with the rails and is configured to move acrossthe surface of the photosensitive polymer layer.
 53. The systemaccording to claim 52, wherein the carriage and rails are configured tomove the laser in the longitudinal direction and the clamp is configuredto move the photosensitive polymer layer in the latitudinal direction.